So a Bugbear with a polearm has a 15ft reach(Long-Limbed) each creature who enters its reach gets attacked(Polearm Master) and they must stop if hit(Sentinel) they would have to take 3 turns to be able to attack the character or anyone by/behind them stopping every time they move 5ft(Tunnel Fighter)providing the attacks hit. This seems really OP.
When you take the Attack action and attack with only a glaive, halberd, or quarterstaff, you can use a bonus action to make a melee attack with the opposite end of the weapon. This attack uses the same ability modifier as the primary weapon. The weapon’s damage die for this attack is a d4, and the attack deals bludgeoning damage. While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter the reach you have with that weapon.
Tunnel Fighter (Unearthed Arcana):
You excel at defending narrow passages, doorways, and other tight spaces. As a bonus action, you can enter a defensive stance that lasts until the start of your next turn. While in your defensive stance, you can make opportunity attacks without using your reaction, and you can use your reaction to make a melee attack against a creature that moves more than 5 feet while within your reach.
You have mastered techniques to take advantage of every drop in any enemy’s guard, gaining the following benefits:
When you hit a creature with an opportunity attack, the creature’s speed becomes 0 for the rest of the turn. Creatures provoke opportunity attacks from you even if they take the Disengage action before leaving your reach. When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature